The invention relates to a method of demodulating a signal having a main component and harmonics.
Such methods are currently used within appliances for receiving electromagnetic radio signals, which appliances comprise in general: (a) an antenna system enabling the reception of an electromagnetic signal and the transformation thereof into an electronic signal, called radio signal, having a radio frequency, (b) a frequency conversion device designed to deliver a signal, called intermediate signal, which has an intermediate frequency and is representative of the radio signal, and (c) a device for demodulating the intermediate signal.
In known receiver devices, the intermediate signal has a main component and harmonics and is submitted to a bandpass filtering before being demodulated, which filtering has the effect of suppressing the harmonics. The main component is thus selected because it has a higher amplitude and is on that account considered more significant than the harmonics, so that it is this main component which is the object of a demodulation.
One of the major trends in the electronics industry is the effort to reduce the size of manufactured appliances, with a triple aim of reducing the production cost, reducing the power consumption, and increasing the user comfort by decreasing the weight and bulkiness of the appliances thus miniaturized.
The frequency converter and the demodulation device are accordingly more and more realized in the form of integrated circuits, indeed joined together in a single integrated circuit. The realization of the frequency converter in an integrated form requires that the value of the intermediate frequency should be comparatively low. Leakage effects take place between the input and the output of the demodulator, which lead to the appearance of a parasitic component in the output signal of the demodulator with the frequency of its input signal. If the variation ranges of the frequencies of the input and output signals of the demodulator overlap, it is impossible to filter out the parasitic component without affecting the demodulated signal. This parasitic component substantially degrades the performance of the appliance, in particular if the received radio signal is of low power.
In a radiotelephone complying with the DECT standard, for example, including an integrated frequency converter, the central intermediate frequency is chosen to be equal to 864 kHz, which implies that the instantaneous frequency of the input signal of the demodulator varies between 400 and 1300 kHz, while the variation range of the frequency of the demodulated signal is from 0 to 700 kHz. Now since the leakage effects are mainly due to imperfect attunements between various elements which form the demodulator, and are strongly linked to imperfections in the manufacturing process used for producing the demodulator, the amplitude of these leakage effects may vary considerably from one demodulator to the next, which leads to random drops in the efficiency of the manufacture of the known demodulators.
The invention has for its object to remedy these disadvantages by proposing a demodulation method that ensures that the variation ranges of the frequencies of the input and output signals of the demodulator do not overlap.
According to the invention, therefore, a method of demodulating a signal having a main component and harmonics basically comprises: (a) a selection of one of the harmonics, and (b) a demodulation of the selected harmonic.
According to this method, the parasitic component introduced by the demodulator into the demodulated signal will have the frequency of the selected harmonic as its central frequency. To take again the example of the DECT type radiotelephone referred to above, the central frequency of the third harmonic, which is the harmonic closest to the main component of the intermediate signal, is of the order of 2.6 MHz, with its instantaneous frequency varying between 1.3 and 3.9 MHz.
The method according to the invention thus enables the elimination of the parasitic component, for example by a low-pass filtering, without affecting the data transmitted by the demodulated signal.
Generally, the invention may be implemented in all types of applications in which a demodulator is likely to generate leakage effects. The invention thus also relates to a demodulation device for a signal having a main component and harmonics, the device comprising: (a) a bandpass filter designed to receive said signal and to deliver a filtered signal, and (b) a demodulator designed to receive the filtered signal and to deliver a demodulated signal, which device is characterized in that the bandpass filter has a characteristic which is adjusted around a central frequency of one of the harmonics.
In one of its most advantageous embodiments, the invention also relates to a receiver for electromagnetic signals, the receiver comprising: (a) an antenna system enabling the reception of an electromagnetic signal and the transformation thereof into an electronic signal, called radio signal, having a radio frequency, (b) a frequency conversion device designed to deliver a signal, called intermediate signal, which has an intermediate frequency and is representative of the radio signal, and (c) a device for demodulating the intermediate signal, characterized in that the demodulation device includes the bandpass filter and the demodulator of the previous paragraph.